Method of diffusion



Sept. 7, 1965 J. o. M CALDIN 3,205,102

METHOD OF DIFFUSION Original Filed Nov. 22, 1960 2 Sheets-Sheet 1 Fig.1.

James O. McCaldin,

INVENTOR.

Wham-1; 91

A T TOR/V5 Y.

Sept. 7, 1965 J. o. MCCALDIN 3,205,102

METHOD OF DIFFUSION Original Filed Nov. 22. 1960 2 Shets-Sheet 2 James0. Mc Cu ldi n,

INVENTOR.

wmmyd.

ATTORNEY.

United States Patent 0 3,205,102 METHOD {BF DIFFUSION James 0. McCaldin,Los Angeles, Calif., assignor to Hughes Aircraft Company, Culver City,'Calif., a corporation of Delaware Continuation of application Ser. No.70,951, Nov. 22, 1960. This application Sept. 16, 1963, Ser. No. 309,9176 Claims. (Cl. 148'189) This is a continuation of my application S.N.70,951, filed November 22, 1960, entitled Method of Diffusion, nowabandoned.

This invention relates to diffusion techniques for use in themanufacture of semiconductor devices, such as diodes and transistors.More particularly, the invention relates to the gaseous diffusion ofconductivity-type-determining materials into semiconductive bodies forthe purpose of establishing in such bodies areas therein of a desiredtype of conductivity to a predetermined extent.

It is well known that semiconductor rectifying and/or amplifying devicesare provided by the establishment of one or more rectifying junctions orbarriers in semiconductor bodies. Such junctions are formed betweenadjacent regions of different or opposite types of conductivity in asemiconductor body. These junctions are usually nominated PN junctionsin the semiconductor art although rectifying junctions may beestablished between regions of different rather than oppositeconductivity. Thus, for example, P-P+ junctions or N-N- junctions may beformed. The term N-type conductivity is intended herein to refer to theconduction of current in a semiconductor body by means of electronsavailable in such body due to the presence of a material therein capableof giving up electrons, such a material therefore being called a donorimpurity. The term P-type conductivity is intended herein to refer tothe conduction of current in a semiconductor body by means of holespresent in such body due to the presence of a material therein capableof borrowing or accepting electrons, such a material therefore beingcalled an acceptor impurity. Such junctions as PP+ junctions may beestablished by adjacent regions having the same or likeconductivity-typedetermining impurities disposed therein but ofdifferent concentrations and hence resistivities.

The introduction of such conductivity-type-determining impurities into asemiconductor body may be accomplished by several methods but thepresent invention relates to a process of increasing value and interestbecause of the excellent opportunity for controlling the amount anddistribution of the impurity afforded thereby. This is the process ofdiffusing atoms of a conductivity-typedetermining material (called thediffusant in this process) into a substrate or semiconductor bodywithout significant melting of the substrate. By way of contrast, fusionand alloying describe the introduction of an impurity agent into asubstrate by melting at least a portion of the substrate for the mixingof the impurity agent and substrate.

The invention has special application to processes which involve gaseousdiffusion into silicon where a solid silicon body is exposed to a vaporwhich includes a conductivitytype-determining impurity in one of itsoxidation states, including the elemental form, for the introduction ofthe conductivity-type-determining impurity as the diffusant into thesilicon body without significant melting of the silicon body. It will beconvenient hereinafter to describe as a conductivity-type-determiningimpurity, any element or compound which includes a component which willserve either as an acceptor or as a donor in a semiconductive body.

Heretofore two different systems have been in practice to accomplish thediffusion of a conductivity-type-determining impurity into asemiconductive body. In one system, hereinafter called the closed tubesystem, the semiconductive body is enclosed within a vessel with theconductivity-type-determining impurity and the vessel is sealed off fromthe atmosphere. Thereafter the vessel is heated to an appropriatetemperature to achieve diffusion of the impurity into the semiconductivebody. Such a system is disclosed in US. Patent No. 2,849,343 to F. A.Kroger et al.

The other diffusion system referred to hereinabove is disclosed in US.Patent No. 2,873,222 to L. Derick et al. This system may be called thegas flow or open system since the semiconductive body is disposed intoan essentially open-ended tube and gas containing aconductivitytype-determining impurity is caused to flow through the tubeand over the semiconductive body. As shown in the referenced patent toDerick et al. the source of vapor containing theconductivity-type-determining impurity may be external to the tube andpumped thereto and therethrough, possibly with an inert carrier gas,such as argon. Alternately the source may be positioned within the tubeupstream of the semiconductive body, and heated to produce theconductivity-type-determining vapor which is carried downstream to thesemiconductive body by a carrier gas.

One of the advantages of the closed tube system is that once purity ofthe interior of the container or tube and its contents has been achievedit is maintained throughout the diffusion process. In addition, theclosed tube system is one which during the diffusion process isindependent of kinetic changes, which is to say that the chemicalprocesses involved in forming the diffusant vapor and in diffusing theimpurity into the semiconductive body constitute a system isolated fromtheir surroundings and may be carried out relatively undisturbed whichresults in a large measure of control over the difiusion process.

On the other hand there are serious drawbacks in the closed tube systemwhich a manufacturer of diffused semiconductor devices must take intoconsideration. For one thing the cleansing and purification of theclosed tube or capsule is difficult and tedious to accomplish. Inaddition a tube or capsule may be used only once since it is destroyedwhen opened at the end of the diffusion run. Also the tube or capsulemust be of fusible or sealable material which limits the choice ofmaterials which may be used for the capsule.

The open tube or gas flow system permits re-use of the apparatusinvolved any number of times and the choice of materials available forthe apparatus is wider than in the case of the closed tube system sincethe limitation of employing sealable materials is not present. However,as practiced heretofore, the gas flow system has several disadvantagesprincipally due to the necessity of employing a flowing stream of gas.Because there are large temperature differences within the tube, largeconvective gas flows are present; many diffusants are thereforenonuniformly distributed in the doping atmosphere which means unevendiffusion or doping of the semiconductive body. In addition, erosion ofthe surface of the semiconductive body often occurs especially whenlarge gas velocities are employed. As described in Patent No. 2,802,760to Derick et al. it was, in part at least, erosion or pitting duringdiffusion which led to the suggested use of an oxide film over thesurface of the semiconductive body during diffusion. It will also beappreciated that the use of a relatively large amount of flowing gaswill render temperature control within the system more difficult toachieve.

It has also been discovered that one of the reasons why uniformconcentrations of the diffusant in the doping atmosphere cannot beestablished initially and maintained is because the diffusant isdepleted from the diffusing atmosphere by reaction with the apparatus orcontainers therefor. It has been noticed that during the initial periodsof diffusion, the semiconductive body may be doped unevenly by thediffusant but that subsequently the doping became relatively uniform asthe absorbing or reacting diffusion apparatus became saturated and nolonger depleted the diffusant from the doping atmosphere.

An object of the invention is to provide an improved method fordiffusion whereby the advantages of both closed and open diffusionsystems may be realized without the disadvantages of either. To thisend, a semiclosed diffusion vessel having a pair of small opposedopenings therein is disposed within a larger container or tube which isadapted to permit a stream of gas to flow therethrough as in thepreviously described open or gas flow system. The semiconductive body tobe doped by the diffusant is disposed in the semi-closed vessel (whichmay be referred to as the difiusion vessel or chamber) together with adiffusant source. Both the diffusion vessel and the larger containertherefor (hereinafter called the furnace tube) are adapted to be heated.The diffusant source within the diffusion vessel may be provided thereinseparately from the diffusion chamber or the diffusion chamber itselfmay constitute the diffusant source as will be described more fullyhereinafter. During diffusion the diffusion vessel is heated and astream of gas, which may be inert, is caused to flow through the furnacetube and around the diffusion vessel which is thus effectivelysealed-off from the open atmosphere as in the aforedescribed closed tubesystem. By controlling the velocity of the gas stream in the furnacetube the gas stream can be made to flow through or substantially onlyaround the diffusion chamber.

The invention will be described in greater detail by reference to thedrawings wherein:

FIGURE 1 is an elevational view in section of diffusion apparatussuitable for the practice of the method of the present invention;

FIGURE 2 is an elevational view in section of the upper portion of asemi-closed vessel suitable for use in diffusion apparatus according tothe method of the present invention;

FIGURE 3 is an elevational view in section of the lower portion of asemi-closed vessel suitable for use in diffusion apparatus according tothe method of the present invention;

FIGURE 4 is a cross-sectional view of the upper portion of thesemi-closed vessel shown in FIGURE 2 taken along the line 4-4 thereof;

FIGURE 5 is a cross-sectional view of the lower portion of thesemi-closed vessel shown in FIGURE 3 taken taken along the line 5-5thereof; and

FIGURE 6 is a cross-sectional view of a semi-closed vessel with theupper and lower portions thereof in place for use in diffusion apparatusaccording to the method of the present invention.

According to the present invention the semi-conductive body into whichit is desired to diffuse a conductivitytype-determining impurity isdisposed in an atmosphere containing the impurity which atmosphere ismaintained out of contact with the ambient room atmosphere by means of asecond atmosphere of controlled content and purity which envelops orsurrounds the impurity-containing atmosphere. Communication between theimpuritycontaining or diffusing atmosphere and the enveloping orisolating atmosphere is maintained. The vessel which contains thesemiconductive body is not integrally closed or sealed-off against theambient atmosphere and is in fact adapted to be readily opened to permitthe loading and unloading thereof. In addition, the vessel may beprovided with one or more openings so as to permit the introductionthereinto of any desired atmosphere at any time prior to, during, orafter actual diffusion of an impurity into the semiconductive body orother treatment thereof. Since this vessel is not sealed-off against theambient atmosphere and is in fact provided with openings to permitcommunication with the interior of the vessel, the effect of asealed-off vessel is achieved according to the present invention bysurrounding this vessel and its semi-enclosed atmosphere and contentswith an isolating atmosphere. This isolating atmosphere may beconveniently provided in apparatus, which will be described in greaterdetail hereinafter, wherein the isolating atmosphere is caused to flowgently through an additional vessel or furnace tube which contains theaforementioned semi-closed vessel. At any time by changing the velocityof the isolating atmosphere entry thereof into the semi-closed vesselcan be provided.

According to the invention, and as an example of the method thereof, asemiconductive body of silicon, for example, may be doped by diffusionas follows. As shown in FIGURE 1, a silicon body 2, which may be acircular wafer about 15 mils thick and having a diameter of 0.75 inch,after having been suitably cleansed and prepared for diffusion bytechniques known in the art, is disposed in a diffusion vessel 4 ofquartz, for example. This vessel is shown in greater detail in FIG- URES2 to 6 inclusive. The diffusion vessel may be made of other refractoriessuch as graphite or alumina, for example. As shown the diffusion vessel4 comprises an elongated boat portion 6 having an upper lid portion 8,the two portions being adapted to be brought together to form asemi-closed chamber 10 defined by the cavity portions 12, 12, providedtherein, respectively. One end of the boat 6 is provided with a passageor port 14 therethrough. The end of the lid 8 opposite to the endthereof which is disposed over the port-containing end of the boat 6,when the lid is in place on the boat, is likewise provided with apassage or port 16. Thus when the lid 8 is placed over the boat 6, achamber 10 is formed by the cavities 12, 12' thereof and each end of thevessel is provided with an opening or port. In addition, the boat 6 isprovided with one or more slots 17 in the bottom of the cavity portion12 running along a portion of the length thereof to receive one or moresemiconductive bodies to be processed.

A source of difusant impurity 20 in a small crucible 22 may be placedwithin the vessel 4 with the semiconductive body or bodies 2. The lid 8is then positioned in place over the boat 6 and the assembly is insertedwithin a tubular container 24 which itself is contained within a furnace26, the ends of the container 24 protruding from the furnace 26. Forconvenience'the container 24 is referred to as the furnace tube. Thefurnace tube 24, which may be of quartz, has one exterior end connectedto a source of inert gas (not shown)which may be argon, for example. Theother exterior end of the furnace tube 24 may be vented to theatmosphere or provided with a connection so as to convey any dischargedgas to a collection chamber, likewise not shown. The furnace 26comprises a relatively thick-walled container 28 of heat insulatingmaterial having an internal cavity 30 in which is disposed a heatingelement 32.

The furnace tube 24 and the diffusion vessel 4 are disposed in thefurnace 26 so that the diffusant source 20 and the semiconductive body 2are all positioned within the heating region of the furnace.

Prior to actually diffusing the impurity into the semiconductive body 2,the entire diffusion apparatus comprising the furnace tube 24 and thediffusion vessel are purged of any undesirable atmosphere by causing aninert gas such as argon to flow therethrough at a veloclty of about 20cm. per second or more. The inert gas at this velocity will flow notonly around the semi-closed diffusion vessel 4 but will also enter thisvessel by means of the port 14 and exit therefrom by means of the port16 so as to carry with it any undesired gases. These gases are thenexpelled from the end of the furnace tube 24. This purging operation maybe continued for about minutes in order to satisfactorily remove all ofthe undesired gases. Thereafter the ffow of the inert gas into the tube24 is reduced to a velocity of about 0.5 cm. per second or less. At thisreduced or zero fiow rate only a small amount or none of the inert gaswill enter the semi-closed diffusion vessel 4. The diffusion vessel 4however is effectively isolated from the ambient atmosphere since thevessel 4 and its openings are surrounded by the inert gas.Communication, however, between the interior of the diffusion vessel 4and the atmosphere therein with the surrounding inert atmosphere isstill available by means of the ports 14 and 16. Thus to all intents andpurposes, the diffusion vessel 4 is an open vessel when it is desired toload or unload the same or when it is desired to flush the same with apurging inert gas. On the other hand, the diffusion vessel 4 is a closedvessel during the diffusion operation.

Actual diffusion is accomplished by energizing the heating element 32 soas to heat the silicon body 2 to a temperature of about 1100 C. At thesame time the source of diffusant impurity is heated to about the sametemperature so that the impurity is rendered vaporous and dispersedthroughout the diffusing chamber 10 into contact with the silicon body2. It will be appreciated that the desired depth of diffusion isdetermined by the temperature and by the duration of the diffusiontreatment. Thus a P-type region in silicon may be satisfactorilyestablished to a depth of about 2 microns by diffusing boron into asilicon body for about 1 hour at a temperature of about 1150 C.

After a suitable diffusion depth has been obtained, the heating element32 is de-energized and the flow of inert gas into the furnace tube 24may be increased to a velocity of about 20 cm. per second so that theinert gas enters the diffusion vessel 4 and sweeps all of the diffusingatmosphere therefrom. When the apparatus has sufficiently cooled, thediffusion vessel 4 may be withdrawn from the furnace tube 24 and openedup to permit removal of the diffused semiconductive body 2. It will beappreciated that the semiconductive body 2 initially may have beenintrinsic, that is not doped, in which case diffusion of either a donoror acceptor impurity will have resulted in forming an N-type or a P-typeregion therein with a rectifying junction between the N- or P-typeregion and the intrinsic bulk region where no diffusion occurred. On theother hand, a previously doped semiconductive body may have beenemployed and an impurity imparting the opposite type of conductivity mayhave been diffused so as to produce a body having'adjacent P- and N-typeregions with a PN rectifying junction therebe-tween.

As a typical example, the semiconductive body 2 may be initially N-typesilicon, the N-type conductivity being due to the incorporation of adonor impurity such as arsenic therein. A P-N junction in this body isformed by diffusing an acceptor impurity therein. A suitable acceptorimpurity may be boron, for example. To obtain the diffusion of boron,the diffusant source 29 may comprise B 0 in powder form which whenheated to a temperature of at least 1000 C. forms a vapor containingboron which will readily diffuse into silicon.

In a preferred method for carrying out the diffusion of boron intosilicon, the diffusant source 20 of boron is first placed in thediffusion vessel 4 without the silicon body 2 being contained therein.The diffusion vessel 4 is then inserted into the furnace tube 24 in thefurnace 26. After purging the diffusion chamber 10 with an inert gas, asdescribed previously, the diffusant source 20 is heated to form aboron-diffusing atmosphere. Depending upon the temperature and the sizeand shape of the diffusion chamber 10, this boron diffusing atmosphereis maintained for a period of time sufiic-ient to permit the interior ofthe diffusion chamber to become thoroughly saturated with boron. In aquartz diffusion vessel having a volume of about 1000 cc., thissaturation was attained after 30 minutes at a temperature of 1250 C. Ithas been discovered that the irregularity in the diffusion of boron intosilicon during the initial stages of diffusion has occurred in the pastbecause significant amounts of boron were absorbed by the apparatusitself. After saturation of the diffusion chamber with boron, thediffusion vessel 4 is withdrawn from the furnace tube 24 and the siliconWafer 2 is inserted therein. At this point, the diffusant source 20 maybe removed if desired, it having also been discovered that the saturateddiffusing chamber 10 will serve as a satisfactory source of diffusant ifthe foregoing saturation technique has been employed. After re-insertionof the diffusing vessel 4 into the furnace tube 24, the system may againbe purged with inert gas as described previously and diffusion isaccomplished in a similar manner as described.

A donor impurity may be diffused into a semiconductive body by similartechnique although there appears to be less tendency of the apparatus toabsorb donor impurities. To accomplish the diffusion of a donor impurityinto the semiconductive body 2, the diffusant source 20 may include, forexample, about five grams of Sb O which is heated to a temperature of1000 C. whereby antimony is diffused into the semiconductive body.

What is claimed is:

1. The method of treating a semiconductive body comprising the steps of:establishing an atmosphere containing a conductivity-type-determiningimpurity in a vessel in which said semiconductive body is to be treatedbut before disposing said semicondutive body in said vessel, wherebysaid vessel absorbs said conductivity-typedetermining impurity,thereafter disposing said semiconductive body in said vessel,establishing an atmosphere of inert gas around said vessel, and heatingsaid vessel to establish a diffusant atmosphere containing said absorbedconductivity-type-determining impurity within said vessel to therebydiffuse said impurity into said semiconductive body from said diffusantatmosphere.

2. The method of treating a semiconductive body of silicon comprisingthe steps of: establishing an atmosphere containing an acceptor impurityin a vessel in which said silicon body is to be treated but beforedisposing said silicon body in said vessel, whereby portions of saidVessel become saturated with said acceptor impurity, thereafterdisposing said silicon body in said vessel, establishing an atmosphereof inert gas around said vessel, and heating said vessel to establish adiffusant atmosphere containing said acceptor impurity within saidvessel to thereby diffuse said acceptor impurity into said silicon bodyfrom said diffusant atmosphere.

3. The method according to claim 2 wherein said acceptor impurity isboron.

4. The method of treating a semiconductive body comprising the steps of:flowing a first atmosphere which includes substantially only an inertgas at a first velocity around and through a vessel containing asemiconductive body, flowing said first amosphere at a reduced velocityaround said vessel, and establishing a second atmosphere containing aconductivity-typedetermining impurity in '3 said vessel and diffusingsaid conductivity-type-determining impurity into said semiconductivebody from said second atmosphere While maintaining said first atmosphereflowing at said reduced velocity around and in contact with said vesseland diifusing said impurity from said second atmosphere into saidsemiconductive body.

5. The method of treating a semiconductive body con1- prising the stepsof: flowing a first atmosphere which includes substantially only aninert gas at a first velocity around and through a vessel containing asemiconductive body and a conductivity-type-determining impurity,flowing said first atmosphere at a reduced velocity around said vessel,and establishing a second atmosphere containing saidconductivity-type-determining impurity in said vessel and diffusing saidconductivity-type-determining impurity into said semiconductive bodyfrom said second atmosphere While maintaining said first atmosphere atsaid reduced velocity around and in contact with said vessel.

6. The method according to claim 5 wherein said second atmosphere isestablished by heating said conductivity-type-determining impurity insaid vessel.

References Cited by the Examiner UNITED STATES PATENTS 3,066,052 11/62Howard 148189 BENJAMIN HENKIN, Primary Examiner.

5. THE METHOD OF TREATING A SEMICONDUCTIVE BODY COMPRISING THE STEPS OF:FLOWING A FIRST ATMOSPHERE WHICH INCLUDES SUBSTANTIALLY ONLY AN INERTGAS AT A FIRST VELOCITY AROUND AND THROUGH A VESSEL CONTAINING ASEMICONDUCTIVE BODY AND A CONDUCTIVITY-TYPE-DETERMINING IMPURITY,FLOWING SAID FIRST ATMOSPHERE AT A REDUCED VELOCITY AROUND SAID VESSEL,AND ESTABLISHING A SECOND ATMOSPHERE CONTAINING SAIDCONDUCTIVITY-TYEP-DETERMINING IMPURITY IN SAID VESSEL AND DIFFUSING SAIDCONDUCTIVITY-TYPE-DETERMINING IMPURITY INTO SAID SEMICONDUCTIVE BODYFROM SAID SECOND ATMOSPHERE WHILE MAINTAINING SAID FIRST ATMOSPHERE ATSAID REDUCED VELOCITY AROUND AND IN CONTACT WITH SAID VESSEL.